Flexible heating element having a layer of fixing varnish

ABSTRACT

A heating element has a flat, flexible strip of electrically insulating material around which is wound a conductor wire. A layer of fixing varnish covers the turns of the conductor wire and the flat strip. Two clips attached to the ends of the strip provide electrical contact with the conductor wire and serve to attach two electricity connection cables. The heating element can be bent without this affecting its ohm value since the fixing varnish prevents the turns of the conductor wire from moving when the strip is being bent.

BACKGROUND OF THE INVENTION

This invention relates to a heating element suitable for application onthe rigid surface of objects in order to transfer heat to them bypassing an electric current through the heating element.

Such heating elements can be used to heat all kinds of industrial ordomestic objects, such as: feeding bottles, fish tanks, car steeringwheels, window cleaners, locks, hair rollers, evaporators for airfresheners, insect repellents, balm products etc. In each case theheating element must adopt the outer shape of the body to be heated or,in the case of evaporators, the shape giving the most suitableheat-transfer geometry, e.g. a toric shape surrounding the wick of theevaporator without touching it.

Flexible heating elements have been described previously, in patentsEP-411542, WO 90/06010 and EP-245776. None of them ensures that the ohmvalue of the heating element remains constant during bending, andmoreover none is produced by a procedure that is cheap enough to makeits application viable in the domestic products mentioned above.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide aflexible heating element which avoids the disadvantages of the priorart.

In keeping with these objects and with others which will become apparenthereinafter, one feature of the present invention resides, brieflystated, in a heating element in which a safe heating element, free fromany overheating that might cause spontaneous ignition and even fires. Aheating element of this kind is particularly appropriate for domesticapplications, and achieving it is one of the objectives of thisinvention.

Another object of this invention is to achieve a controlled, steady flowof heat, for which the resistance value in ohms must remain constantduring bending.

Lastly, arriving at an essentially inexpensive product is sought, onewhose value would be only a small fraction of the value of the productit is to heat, in such a way that the resulting device may remaincompetitive in the market.

When the heating element is formed in accordance with the presentinvention, it solves the above mentioned problem and attains theobjective set consists of a flat strip of electrical insulating materialthat is heat-resistant and highly flexible, a conductor wire woundcontinuously around the flat strip with its longitudinal ends connectedto a power supply circuit, and a layer of fixing varnish whichcompletely covers the strip and the turns of the conductor wire, all soarranged that the layer of fixing varnish holds down firmly each turn ofthe conductor wire in position on the strip, preventing it from slippingon the strip during any bending to which the heating element may besubjected and preventing electrical contact between the turns of theconductor wire during any such bending, in such a way that the ohm valueof the heater element remains constant in all cases.

According to another characteristic feature of the invention, a metalclip is provided at each end of the strip, pressing mechanically on itand penetrating the layer of varnish to make electrical contact withseveral end-turns of the conductor wire at the ends of the heatingelement, and each having a cable connecting them to the electricitysupply circuit.

According to another characteristic feature of the invention, each ofthe metal clips consists of a flat metal plate with its ends folded intoa U shape, the central portion being fitted on one face with smallprotuberances which are designed to be plastically deformed, the clipbeing arranged in such a way that the two side portions of the U shapefit around one end of the heating element and grip it, while the twoprotuberances penetrate the layer of varnish and provide electricalcontact with the corresponding connection cable.

According to another characteristic feature of the invention, one ofthese clips is provided with a ring on its central portion, preferablypress-formed, into the eye of which one of the connection cables may beinserted, it then being possible to arrange plastic deformation of thisring making it press down on and clamp firmly in place the connectioncable in order to ensure mechanical and electrical contact between thecable and the clip.

According to another characteristic feature of the invention, theheating element is contained inside a body provided with a hole in itscentre with runs through it, the body consisting of at least oneinternal piece and one external piece which fit together with some spacebetween them for containing the heating element, in such a way that thelatter, when the internal piece is inserted inside the external piece,is hermetically sealed in between the two pieces, which are fixedtogether by coating them with a sealing material on the outside in orderto make up this body, it also being provided with holes for taking theconnection cables.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings attached illustrate, as non-limiting examples, anembodiment of this invention.

FIG. 1 shows a view of the heating element of this invention instretched-out form.

FIG. 2 is a blown-up detail of portion II of FIG. 1.

FIG. 3 shows a front elevation of one of the clips providing contactbetween the heating element and the cable.

FIG. 4 shows a perspective view of the clip shown in FIG. 3.

FIGS. 5 and 6 are two diagrams, in perspective, of an application of theheating element of this invention in an encapsulated heating device, inwhich the capsule is straight-edged, though it could also be curved indesign.

FIG. 7 shows a general view, in perspective, of a thermal fuse which canbe used as a safety device in a heating element according to thisinvention.

FIGS. 8 and 9 are two perspective views of two embodiments of theheating element of this invention, one arranged in a helicoidal mannerand the other in a circular manner.

FIG. 10 is a perspective view of the assembly stage of a one-piece bodyin which the heating element of this invention is contained.

FIG. 11 is a perspective view of the one-piece body of FIG. 10 afterassembly.

DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION

As can be seen from the drawings, the heating element (1) consists of aflat, flexible strip (2) of electrically insulating and heat resistantmaterial, upon which is wound, through a continuous turning process, avery fine conductor wire (3).

As can be seen in FIG. 2, the strip (2) wound on the conductor wire (3)has a fine layer of fixing varnish (14) applied to it. It is applied byimmersing the strip with its winding in a bath of liquid varnish.Afterwards, after drying and/or curing the varnish layer (14), itadheres strongly to the strip (2) and to the turns of the conductor wire(3).

After drying and/or curing, this varnish has good heat resistance forthe working temperatures of the heating element (1) and is sufficientlyflexible for the varnish layer (14) not to have any significant adverseeffect on the flexibility of the wound strip (2). The varnish layeradheres to the strip (2) and to the turns of the conductor wire (3) insuch a way that the fixing varnish (14) retains the turns (3) inposition on the strip (2) during any bending of the strip (2),preventing the wire (3) from moving around on the strip (2). Thus theohm value of the heating element (1) remains constant throughout anybending of the strip (2).

A power connection cable (4, 5) is fitted to each end of this woundstrip (2). These cables (4, 5) make electrical contact with theconductor wire (3) owing to U-shaped clamps (6, 7) which provideelectrical and mechanical contact between the wire (3) and the cables(4, 5).

Each of these clips has two side portions (8, 9), and a central portion(10) to which the side portions (8, 9) are joined, as can be seen inFIGS. 3 and 4.

The central portion (10) has a ring (11) and two protuberances (12, 13)on the side opposite the ring (11). The end of a connection cable (4, 5)is passed through the eye of the ring (11), and the ring (11) is laterplastically deformed until good electrical contact and good mechanicalfastening are achieved.

The side portions (8, 9) are turned in towards the side opposite thering (11), wrapping around the ends of the wound strip (2), until aplastic deformation occurs causing attachment to the strip.

During this procedure, the central portion (10) of the clip (6, 7)presses strongly against one of the faces of the strip (2). As a resultof this pressure, the protuberances (12, 13) break the layer of varnish(14) and penetrate through it to make contact with the conductor wire(3). It is thus these protuberances (12, 13) that provide electricalcontact between the heating element and the clips.

In this way an element is achieved that can be bent with no change inits ohm value, and that is highly cost-saving.

Hereunder are described, as non-limiting examples, several practicalapplications of the heating element (1).

Firstly, in very general terms, the heating element (1) can be appliedto the surface of an object to which it is wished to transfer heat.Thanks to the flexibility of the heating element (1), it can be made toadhere to the surface of the object with the aid of a glue. When theelement is stuck onto the object, a layer of silicone or some otherinsulating material is applied to cover the wound strip (2) and theclips (6, 7) completely. With this arrangement, applying an electriccurrent to the ends of the connection cable suffice for theheat-transfer process to the selected object to begin.

One possible application of the heating element (1) is a heatingappliance (20) such as the one shown in FIGS. 5 and 6. Here the heatingelement (1) is encased in a capsule (15) that is electrically insulatedand made of a material that can withstand the working temperatures ofthe heating element (1). In the example shown in FIG. 5, the capsule isof a generally straight-edged shape, though it could be of any curved,flat or three-dimensional shape, since the flexibility of the heatingelement (1) enables it to adapt to any curved form, as has previouslybeen noted.

The capsule (15) is in the form of a box that is open on top. Alongitudinal divider (16), made of the same material as the rest of thebox, divides the interior of the box into two parallel cavities (18,19). This longitudinal divider (16) has an opening (17) at one end.

The heating element (1) is arranged in one of the cavities (18) of thecapsule (15), and it takes the shape (straight in this case) of thecapsule's geometry. One of the connecting cables of the heating element(1) goes out of the capsule (15) through its open top.

The other connection cable (5) of the heating element (1) remains insidethe capsule (15) and connects cavity (18) with cavity (19) by passingthrough the opening (17) provided for this purpose.

The cavity (18) that contains the heating element (1) is then filledwith a liquid product that is hardenable and sealing, e.g. an epoxyresin.

The connection cable (5) is in cavity (19), and a thermal fuse (20) isconnected to it (see FIG. 6) by means of a clip of the same sort as theclips (6, 7) of the heating element (1).

The thermal fuse (20) shown in FIG. 6 is made of a metal alloy. Thisthermal fuse (20) is initially straight in form, yet it can beplastically deformed to take on shapes analogous to those of the heatingelement (1), in combination with which it is applied.

It also has two clips (21, 22) similar to the clips (6, 7) of theheating element. Finally, it is fitted with two connection cables (23,24). The layout of this fuse becomes evident on consulting FIG. 6.

The metal alloy from which the fuse (20) is made and its size are chosenso that this fuse (20) comes into operation when for whatever reason thetemperature in the device (25) approaches the temperature at which thehazard of ignition appears. When this occurs the fuse (20) blows andcuts off the power supply, thus cutting out the system. This fuse (20)is a fire-prevention safety device, and enables the heating appliance(25) to be used in domestic applications.

To increase the effectiveness of the fuse, the mid-point of the thermalfuse (20) will be placed in the hottest region of the heating appliance(25), this usually being at the mid section of the heating element (1).

To achieve a good finish for the heating appliance (25), a cover (26) isused, with a small hole (27) for the connection cable (24) to passthrough.

Another possible application for the element (1) is to arrange heattransfer to an object with no contact or adherence between the element(1) and the object to be heated--for example, in evaporators fordomestic use such as those used for evaporating insecticides, airfresheners or balm products. In these evaporators, the liquid containedin a tray or impregnated in a wick can be evaporated by the heattransferred from the heating element (1). However, this heat transfer ismore even and therefore more effective and certain if the heatingelement (1) is arranged so as to encircle the wick or tray at a setdistance. For this purpose the heating element (1) must take on acircular form (FIG. 9) or a helicoidal form (FIG. 8).

One application of this circular form is shown in FIGS. 10 and 11. Inthis embodiment, the heating element (1) is contained inside a body(28), into which the connection cables (4, 5) come through openings (31)provided for them to enter the body (28). The body (28) consists of aninner piece (29) and an outer piece (30), the heating element (1) beingcoiled in on of them and taking on the circular form shown in FIG. 9.The outer piece (30) fits loosely over the inner piece (29), with theheating element (1) being contained and closed in between them. The twopieces are joined together.

For the object to be heated by the heater element (1) to be encircled bythe latter, thus achieving effective heating, the one-piece body (28)has a central opening (32) running through it of a radius sizeappropriate to the object to be heated.

The nature of the invention having been sufficiently described, as wellas the manner of its implementation, it is to be noted that changes ofdetail may be made to all things provided that the fundamental principleis neither changed nor modified, the essential matter for which atwenty-year patent is sought being summarized in the following claims.It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied in aflexible heating element, it is not intended to be limited to thedetails shown, since various modifications and structural changes may bemade without departing in any way from the spirit of the presentinvention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

I claim:
 1. A heating element, comprising a flat strip of electricalinsulating material which is heat-resistant and flexible; a conductorwire having a plurality of turns and wound continuously around said flatstrip, said conductor wire having longitudinal ends connectable to apower supply circuit; and a layer of fixing varnish which completelycovers said strip and said turns of said conductor wire and arranged sothat said layer of fixing varnish holds down firmly each of said turnsof said conductor wire in position on said strip so as to prevent eachturn from slipping on said strip during any bending to which the heatingelement may be subjected and preventing electrical contact between saidturns of said conductor wire during any such bending, in such a way thatan ohm value of the heater element remains constant when the heatingelement is bent.
 2. A heating element as defined in claim 1; and furthercomprising a body provided with a hole in a center, said body includingat least one internal piece and at least one external piece which fittogether with some space between said pieces for containing said flatstrip with said conductor wire and said layer of fixing varnish so thatwhen said internal piece is inserted inside said external piece, saidflat strip with said conductor and said layer are hermetically sealedbetween said pieces, said body being provided with holes for receivingsaid cables.
 3. A heating element as defined in claim 2; and furthercomprising means for fixing said pieces with one another by pressure. 4.A heating element, comprising a highly flexible flat strip of electricalinsulating material which is heat-resistant and flexible; a conductorwire having a plurality of turns and wound continuously around said flatstrip, said conductor wire having longitudinal ends connectable to apower supply circuit; and a layer of fixing varnish which completelycovers said strip and said turns of said conductor wire and arranged sothat said layer of fixing varnish holds down firmly each of said turnsof said conductor wire in position on said strip so as to prevent eachturn from slipping on said strip during any bending to which the heatingelement may be subjected and preventing electrical contact between saidturns of said conductor wire during any such bending, in such a way thatan ohm value of the heater element remains constant when the heatingelement is bent, said flat strip having ends; and further comprising ametal clip provided in each of said ends of said strip so as to pressmechanically on said strip and penetrate said layer of varnish to makeelectrical contact with several end turns of said turns of saidconductor wire, each of said ends of said strip being provided with acable for connecting said strip to a current supply circuit, each ofsaid metal clips having a flat metal plate with ends folded into U shapeand a central portion with one face provided with small plasticallydeformable precuberances, each of said clips being arranged in such away that two side portions of said U shape fit around and grip one endof said strip, while protuberances penetrate said layer of varnish andprovide electrical contact with a respective one of said cables.
 5. Aheating element as defined in claim 4, wherein one of said strips isprovided with a ring arranged on said central portion and having an eyein which one of said cables is insertable, said ring being plasticallydeformable so as to firmly clamp said cable to ensure mechanical andelectrical contact between said one cable and said strip.